The invention relates to a glass composition of silica-soda-lime type colored blue. More particularly, the invention relates to a blue glass composition for the preparation of flat glasses by floating on a bath of molten metal, such as tin (float process), these flat glasses being intended in particular, but not exclusively, to form windshields and side windows situated at the front of a vehicle.
Automobile windows are subject to very strict requirements. As regards optical properties, these requirements are governed by regulations, for example with regard to the light transmission of a windshield or else with regard to the comfort of the user, in particular as concerns the energy transmission.
In addition to the constraints related to the light transmission and to the energy transmission, the windows situated at the front of vehicles also have to meet the wishes of automobile manufacturers as regards the color, in particular relating to the dominant wavelength and to the purity.
Iron is a coloring agent which fully satisfies these requirements. The presence of iron in the form of ferrous ions Fe2+, distinct from ferric ions Fe3+, makes it possible to reduce the transmission of infrared radiation through the glass and therefore to lower the energy transmission. In addition, iron introduces a green coloring which matches well the color of the majority of automobiles.
Nevertheless, the combination of a bluish body and of a green window proves in many cases to be rather unsatisfactory from an esthetic viewpoint. This is why automobile manufacturers wish to have available glasses exhibiting a relatively neutral blue color, that is to say exhibiting a wavelength which is not too low and a purity which is not too high, for a significant level of light transmission and also a moderate energy transmission.
A blue coloring can be obtained simply by adding cobalt oxide to the glass composition. The major disadvantage of this oxide is that it leads to a reduction in the light transmission of the glass, the transmission of infrared radiation through the glass for its part being only very slightly affected.
Another way of coloring the glass blue consists in using iron as sole coloring agent, provided, however, that the redox factor (content of ferrous ions FeO/total content of ferrous ions and of ferric ions Fe2O3) is maintained at a relatively high value, of the order of 50%. Such a high redox factor presents problems with regard to the implementation of the process as the melting of the glass is rendered more difficult, which increases in proportion the risk of seeing the appearance in the glass of inclusions of incompletely melted material, such as silica. In addition, under such reducing conditions, the iron is liable to react with the sulfate used for the refining of the glass in the bath to form iron sulfide, which gives the glass a yellow to brown coloring.
Blue-colored glasses can also be obtained by combining several coloring agents.
In EP-A-0 820 964, a mixture combining iron (0.4 to 1.1%) and cobalt oxide (10 to 75 ppm) is used to form a blue glass having a dominant wavelength varying from 480 to 490 nm and an excitation purity of at least 6%. The proportion of iron in the ferrous state is between 20 and 40%. The coloring effect, related essentially to the presence of cobalt oxide in the glass, is reflected by a very strong blue color.
In EP-A-0 814 064, provision is made to combine iron (0.53 to 1.1%), cobalt oxide (5 to 40 ppm) and optionally chromium oxide (up to 100 ppm) to form blue glasses exhibiting a dominant wavelength varying from 485 to 491 nm and a purity varying from 3 to 18%. The redox is between 0.25 and 0.35.
In EP-A-1 023 245, use is made, as above, of iron (0.4 to 1.0%), cobalt oxide (4 to 40 ppm) and optionally chromium oxide (up to 100 ppm) to form a glass exhibiting a dominant wavelength varying from 485 to 489 nm and an excitation purity ranging from 3 to 18%. This glass is prepared at a redox of between 0.35 and 0.6, which is not a usual redox value for the float process. It is therefore necessary in this case to use specific heating means to melt the composition, as already specified above. This is consequently reflected by an increase in the cost of the glass produced.
To rapidly respond to market demand is a constant concern of manufacturers of glass in general and more particularly of colored glasses for the automobile industry, where the color range is relatively broad. The processes for producing the blue glasses mentioned above are carried out with a total iron content at least equal to 0.4% and/or under relatively high redox conditions. They generally operate with a given glass composition and it is not recommended to adjust the nature or the content of the components participating in the composition of the vitrifiable mixture. This is because any change in the composition of the glass in the furnace requires a transition time during which the glass produced does not have the expected optical properties and the expected coloring. This transition time increases in proportion as the content of coloring agents increases. Furthermore, limiting the content of iron, in particular of ferrous iron, introduces an additional advantage as the energy requirement for melting the glass composition is less, which contributes to reducing the cost of the glass.